1. Field of the Invention
The present invention relates to an audio decoder and an audio decoding method for obtaining audio data by decoding encoded audio data. Specifically, it relates to an audio decoder and an audio decoding method which achieves to improve the sound quality at the time of error occurrence when encoded audio data is decoded in a decoding process.
2. Description of the Related Art
In compressed audio data (MP3, AAC, Dolby Digital, ATRAC and the like) which have become popular lately, data is compressed using combination of methods such as entropy encoding, window function, and orthogonal conversion so as to achieve a higher encoding efficiency than that of linear PCM. Such compressed audio data is replayed by being decoded in a decoder, and there are cases of error occurrence in a recording medium or on transmission paths. Especially, frequency of the error occurrence is high in radio transmission and the like. Thus, it becomes necessary to take measures so that errors are hardly perceived. In general, an error detection code such as CRC code is contained in the audio data to be transmitted, which makes it possible to detect the transmission errors.
When errors are detected through the error detection code as described above, conventionally, the detected audio frame is muted or skipped until reaching the audio frame which can be correctly decoded as the measures to cope with the errors. Also, it is possible to employ a method in which sound is smoothly attenuated by inserting zero to input signal of window function so as to reduce the noise (for example, see Japanese Unexamined Patent Publication 2002-073091).
As another method for making errors hardly perceived, often used is a method in which the audio data which is correctly decoded right before is accumulated in a memory and is repeatedly outputted until the data which can be correctly decoded is received.
However, there are shortcomings in the above-described conventional measures for overcoming errors as will be described below.
The first aspect of the shortcomings is as follows. When the adjacent decoded audio data is used for filling to conceal the error of audio data, the sound becomes discontinuous in between the audio frames, which cause the noise.
The second aspect of the shortcomings is as follows. Error concealing processing is performed by using only the error information within the encoded audio data so that there is only a small selection of factors for determining the error concealing method. Thus, it is hard to take sufficient measures for overcoming errors.
The third aspect of the shortcomings is as follows. The error concealing processing is performed by using only the error information of the audio data adjacent to the audio data which is being decoded currently so that it is impossible to take measures for overcoming errors by predicting the future condition.